At present, digital reception methods are applied to optical receivers used for optical fiber communication systems to thereby achieve a reduction in apparatus cost. In particular, in an optical receiver to which a digital coherent reception method is applied, it is possible to perform dispersion compensation by digital signal processing after an optical signal has been coherently received and converted to an electrical signal. Since it is not necessary to perform dispersion compensation in an optical condition where high cost dispersion compensation fiber and the like is used, a significant reduction in cost for the apparatus cost can be expected.
Specifically, in a digital coherent optical receiver, an optical signal produced by mixing reception signal light with local oscillator light is converted to a digital electric signal by a photoelectric transducer and an AD converter. Then in a digital signal processing circuit to which the digital signal is input, waveform equalization processing and signal discrimination processing and the like for realizing the abovementioned dispersion compensation, are performed. This digital signal processing circuit has a large scale circuit configuration for realizing various functions by arithmetic processing of the digital signal, and often a large part of this is occupied by a circuit block that performs arithmetic processing for waveform equalization.
As a conventional technique related to waveform equalization in the above digital signal processing circuit, there has been proposed a technique in which filtering of the reception signal is performed using a filter or the like having a finite impulse response (FIR) characteristic for example, and a plurality of variable parameters (tap coefficients) serving as weighting for the FIR filters and the like, are appropriately set corresponding to a monitor result of the signal quality, to thereby suppress waveform degradation attributable to waveform dispersion or polarization mode dispersion (for example, refer to Japanese Laid-open Patent Publication No. 2008-35319).
However, in the above conventional optical receiver that realizes waveform equalization using an FIR filter and the like, there is a problem in that the power consumption is large. That is, the influence of chromatic dispersion in the optical fiber transmission line becomes noticeable as the transmission speed of the optical signal is increased. Furthermore, when receiving an ultra high speed optical signal of around 40 Gb/s, the influence of polarization mode dispersion can also not be ignored. Therefore it is necessary to perform compensation for waveform dispersion and polarization mode dispersion at high accuracy. In order to realize such high accuracy dispersion compensation, it is necessary to increase the number of taps of the filter used in waveform equalization, and to optimize each of the tap coefficients according to the state of the optical fiber transmission line. If the number of taps of the filter is increased, the power necessary to effectively operate the filter also increases. Therefore the power consumption of the optical received is increased. In other words, in the conventional optical receiver there is a problem in that, in order to perform waveform equalization at high accuracy by digital signal processing, an increase in power consumption cannot be avoided.